[feature request]: set grace note duration for MIDI playback

[Gilberto Agostinho]

Currently, the MIDI output from LilyPond uses a hard coded value to 
calculate the duration for the grace notes (1/4 of their duration 
according to the documentation or 9/40 according to David Kastrup). It 
would be really nice to allow the users to set this proportion 
themselves, since depending on the type of score one is engraving the 
grace notes can be either too fast or too slow, and the only solution 
right now is to manually create a different score with different values 
for the grace notes in order to produce a good MIDI file.

For the thread which originated this request, see: 
http://lilypond.1069038.n5.nabble.com/grace-note-duration-td177682.html